Typical materials requiring a high degree of surface planarity include a single-crystalline silicon disk called a silicon wafer for producing semiconductor integrated circuits (IC, LSI). In a process for producing IC, LSI etc., the surface of the silicone wafer should be flattened highly accurately in a step of forming an oxide layer or a metal layer in order to provide reliable semiconductor connection with various films used in manufacturing circuits.
Generally, a polishing pad is stuck on a rotatable supporting disk called a platen in the polishing step, while a semiconductor wafer is fixed to a planetary disk called a polishing head. By rotational movement of the two and feeding polishing slurry containing micro particles (abrasive grains) emulsified to a gap between the polishing pad and the semiconductor wafer, the semiconductor wafer is polished and planarized. When the polishing pad moves on the surface of the wafer, the abrasive grains are pushed on the surface of the wafer at the contacting point. Therefore, the processed surface is polished by dynamic frictional effect between the surface of the wafer and abrasive grains. The polishing process is called as CMP polishing process.
As a polishing layer of the polishing pad used for highly accurately polishing, a foamed polyurethane sheet having a void volume of about 30 to 35% is generally used. Polyurethane foam sheet locally has excellent planarity, but the compressibility is within the range of 0.5 to 1.0%, which is small, and the cushioning properties are not sufficient. Thereby, it is difficult to uniformly apply a pressure on the whole surface of the wafer. Therefore, the polishing process is conducted by using a polishing pad comprising a soft cushion layer on the backside of the polyurethane foam sheet as a laminated polishing pad.
A polishing pad using a conventional polyurethane sheet provided with a cushion layer has the following problems.
(1) A nonwoven fabric having continuous pores impregnated with resin is widely used as the cushion layer, but there are problems such as variation among nonwoven fabrics and a change in compression characteristics due to immersion in slurry.
(2) A foamed urethane foam having independent pores comes to be used, but there are still problems such as difficult stabilization of a foamed state in production, significant residual strain resulting from the pores subjected to repeated loading, etc.
In order to solve the problems, as a polishing pad used during CMP process, various laminated polishing pad, such as
(1) a polishing pad comprising a cushion layer having a compression recovery of not less than 90% (formed from a material having rubber elasticity), and a polishing layer laminated on the cushion layer (Japanese Patent Kokai Publication No. 305635/2003),
(2) a polishing pad comprising a cushion layer having a volume modulus of not less than 60 MPa and a tensile modulus of 0.1 to 20 MPa, and a polishing layer having a contact angle to water of not more than 75 degrees, a flexural modulus of not less than 2 GPa and/or a surface hardness in durometer hardness of not less than 80 laminated on the cushion layer (Japanese Patent Kokai Publication No. 59357/2002),
(3) an abrasive component a sheet foam formed from a soft elastomer and abrasive cloth laminated on the sheet foam (Japanese Patent Kokai Publication No. 164307/1995).
In Japanese Patent Kokai Publication No. 305635/2003, a main object is to reduce residual strain for compression load during polishing and reduce the change in compression properties of the polishing pad by using a cushion layer having a compression recovery of not less than 90% (formed from a material having rubber elasticity). Example of the laminated polishing pad, of which the polishing layer is formed from polyurethane resin (which polyurethane is selected is not disclosed), is described. The polishing pad has excellent uniformity within wafer and excellent planarity, but the abrasive rate is very low.
In Japanese Patent Kokai Publication No. 59357/2002, a polishing pad comprising a cushion layer having a volume modulus of not less than 60 MPa and a tensile modulus of 0.1 to 20 MPa, and a polishing layer having a contact angle to water of not more than 75 degrees, a flexural modulus of not less than 2 GPa and/or a surface hardness in durometer hardness of not less than 80 laminated on the cushion layer is disclosed. The excellent planarity is accomplished by using hard resin (such as a laminate using paper and or cloth as base material, or a composition formed by dispersing hydrophilic component in hard matrix resin) as a polishing layer, and the repression of scratch is accomplished by water wetting characteristics. However, the polishing layer has different order of hardness and flexural modulus from that of the present invention, and the cushion layers generally used are exemplified and non-foamed elastomers are disclosed as examples thereof. The polishing pad has excellent uniformity within wafer and excellent planarity, but the abrasive rate is very low.
In Japanese Patent Kokai Publication No. 164307/1995, an abrasive component a sheet foam formed from a soft elastomer and abrasive cloth laminated on the sheet foam is disclosed. As the sheet foam, closed-cell foam formed from natural rubber, synthetic rubber or thermoplastic elastomer is described, but a velour type non-woven fabric is disclosed as the abrasive cloth, which is different from the polishing layer material of the present invention.